1. Field of the Invention
The present invention relates to power control circuitry, and to a method of operating such circuitry, such power control circuitry being arranged to control the connection of a voltage source to an associated circuit.
2. Description of the Prior Art
In a data processing system, it is often the case that many of the circuits within the system spend a significant proportion of the time inactive. However, even when inactive, those circuits can consume significant power due to leakage current. Accordingly, it is known to provide power control circuitry in association with such circuits to disconnect those circuits from a voltage source when they are not being used, and to then reconnect them to the voltage source when those circuits need again to become active.
FIG. 1 is a schematic diagram illustrating a known power control circuitry that can be provided in association with a circuit 40. In this example, the power control circuitry comprises a plurality of PMOS transistors 10, 20, 30 provided in parallel between an operating supply voltage VDD and a voltage line 50 used by the circuit 40, this voltage line 50 also being referred to herein as a virtual VDD line (VVDD). When the circuit 40 is to be disconnected from the power supply VDD, for example when it is entering a standby mode of operation, each of the PMOS transistors 10, 20, 30 receives a power off signal at a logic one level at their gate inputs 15, 25, 35, respectively, which causes those transistors to be turned off, and hence causes the voltage line 50 to be isolated from the operating supply voltage VDD. When the circuit 40 is later required to enter an active mode of operation, then the power off signals are cleared to a logic zero level, thus causing each of the transistors 10, 20, 30 to turn on and pull the voltage line 50 up to the operating supply voltage VDD.
Whilst three PMOS transistors are shown in FIG. 1, it will be appreciated that the number of PMOS transistors provided will vary dependent on embodiment, and indeed in some embodiments only a single PMOS transistor may be required. Further, the voltage line 50 illustrated in FIG. 1 may actually consist of a number of discrete voltage lines, each of which services particular portions of the circuit 40. In addition, the circuit 40 may take a variety of forms. For example, in one embodiment this circuit 40 may take the form of a memory device. Alternatively, the circuit may comprise any logic block within a data processing system, for example a processor core, an arithmetic logic unit (ALU) within a core, a memory controller device, a video controller, etc.
In addition to using such power control circuitry in association with the operating supply voltage, similar circuitry can also be used in association with other voltage sources applied to the circuitry 40, for example the ground voltage. The Article “A Novel Powering-Down Scheme for Low Vt CMOS Circuits” by K Kumagai et al, ULSI Device Development Laboratories, NEC Corporation, 1998 Symposium on VLSI Circuits, Digest of Technical Papers, shows in FIG. 1 a circuit having power control circuitry provided in association with both the operating supply voltage VDD and the ground voltage. In that paper, diodes are also provided in parallel with the transistors of the power control circuitry to limit the change in voltage that can occur on the voltage lines of the circuit, hence ensuring that data can be retained in the standby mode, referred to therein as the sleep mode.
Irrespective of whether the circuit of any particular implementation requires any data holding mechanisms such as the above-mentioned diode mechanism in order to allow the circuit to hold data values whilst it is in the standby mode, a common problem that can occur in any circuit employing power control circuitry is a problem of inrush current when the circuit is later re-connected to the voltage source. Generally, during the shift from the standby state to the active state, a large capacitance may be required to charge up the voltage line 50 to the operating supply voltage VDD and to charge any internal nodes within the circuit 40 that are required to be at the operating supply voltage level when the circuit is in its active state of operation. A large inrush current (voltage surge) may occur in the power supply VDD during this charging period, and this large inrush current may cause voltage drops in the power supply VDD, which might potentially cause other circuits using the power supply VDD to malfunction. It will be appreciated that a similar problem can also be exhibited on other voltage source lines connected to the active circuit using similar power control circuitry, for example the ground line discussed earlier. In particular, any variations in the ground voltage line may again cause other circuits to malfunction.
One way of seeking to reduce the inrush current is described in the article “ChipOS: Open Power-Management Platform to Overcome the Power Crisis in Future LSIs” by H Mizuno et al, 2001 IEEE International Solid-State Circuits Conference. As described therein, when a voltage source is to be applied to a particular logic block, the gates of the power switches provided in power limiter circuitry associated with that logic block are driven with a low slew rate. Accordingly, the voltage on the gates of those power switches is increased relatively slowly so that those switches turn on relatively slowly, and accordingly the time taken to charge up the voltage line of the logic block to the voltage level of the connected voltage source is increased, thereby limiting the peak current. In accordance with the technique described in the article, the gates are driven with a low slew rate by first driving those gates lines with a small driver and only later driving those lines with a large driver. However, whilst such an approach can limit the peak current, and thereby reduce the inrush current, the approach of using simple circuits such as a weak driver to reduce the slew rate of the power switch enable signal provided to the gates of the power switches is very sensitive to manufacturing process variations, and accordingly the amount of current limitation that can be achieved will vary significantly dependent on such process variations. As a result, such an approach does not provide a very reliable technique for reducing inrush current.
The article “Universal Serial Bus (USB) Power Management” by K Lynn, Wescon 97 Conference Proceedings, 4-6 Nov. 1997, Pages 434 to 441, also describes a technique for reducing the inrush current by slowing down the turn-on time of the power switches in the power control circuitry. In accordance with the technique mentioned therein, the power switches are provided with a charge pump which slows the turn-on time to between 1 ms and 2 ms. Whilst the use of more complex analog circuits such as the above-mentioned charge pumps can remove some of the process sensitivity that would be exhibited by the earlier-mentioned technique for limiting the slew rate of the enable signals provided to the power switch gates, their increased complexity adds to the area of the power control circuitry which in turn increases cost.
The article “X2000 Advanced Avionics Project Development of a New Generation of Avionics for Space Applications” by R Blue et al, Aerospace Conference 2003, Proceedings 2003 IEEE, Volume 5, 8-15 Mar. 2003, Pages 5-2303 to 5-2314, describes a number of avionics building block modules, including modules for power distribution and power regulation. A power switch slice is described, and it is mentioned that inrush current control is provided within the power switch slice, but no details of how that is achieved is discussed.
It would be desirable to provide an improved technique for limiting inrush current when employing power control circuitry to control the connection of a voltage source to an associated circuit, which is less susceptible to process variations, but is also less costly to implement than known solutions employing analog circuits such as charge pumps.